1. Field of the Invention
The present invention relates to a device for transmitting optical signals through wavelength division multiplexing using multi-wavelength light provided by a multi-wavelength light source.
2. Description of the Related Art
Presently, Wavelength Division Multiplexing (WDM) communication technology is being put to practical use and the transmission capacity of optical communication is growing significantly (for example, refer to Patent References 1 to 5, below). In the future, it will become necessary to further increase communication capacity due to the progression of the trend towards making all of the transmission paths of subscriber systems fiber-optic.
Patent Reference 1 Japanese Patent Publication No. 2001-197006
Patent Reference 2 Japanese Patent Publication No. 11-261532
Patent Reference 3 Japanese Patent Publication No. 04-336829
Patent Reference 4 Japanese Patent Publication No. 07-177556
Patent Reference 5 PCT International Patent Application Translation Publication No. 2003-50194
FIG. 1A is a block diagram of a WDM transmission system such as this. The WDM transmission system in FIG. 1A is constructed of a terminal station A, a relay station B, and a terminal station C. Station A comprises transmitting units 11-1 to 11-5, receiving units 12-1 to 12-5, and a wavelength multiplexing/separation device 13-1. Station C comprises transmitting units 11-16 to 11-20, receiving units 12-16 to 12-20, and a wavelength multiplexing/separation device 13-4. Station B comprises transmitting units 11-6 to 11-15, receiving units 12-6 to 12-15, wavelength multiplexing/separation devices 13-2 and 13-3, and an electrical ADD/DROP device 14.
The transmitting units 11-1 to 11-20, as shown in FIG. 1B, each comprise a light source 21 of a predetermined wavelength and a modulator 22, and generate optical signals by modulating light from the light source 21 with a transmission data string. The wavelength multiplexing/separation device 13-1 to 13-4 comprises a wavelength multiplexing unit 15, a wavelength separation unit 16, a light transmitting amplification unit 17, and a light receiving amplification unit 18.
The optical signals of each wavelength output from the transmitting units 11-1 to 11-5 of station A are multiplexed by the wavelength multiplexing/separation device 13-1 and transmitted to station B as WDM light. At station B, the received WDM light is separated into optical signals of each wavelength by the wavelength multiplexing/separation device 13-1 and converted into electrical signals by the receiving unit 12-1 to 12-5. The electrical ADD/DROP device 14 divides (DROP) a portion of the received signals and inserts (ADD) other transmission data string.
Next, the WDM light is transmitted from station B to station C in the same way as transmission from station A to station B, and the optical signals of each wavelength are received by the receiving units 12-16 to 12-20 of station C. The transmission procedure from station C to station A is the same as the transmission procedure from station A to station C.
In a WDM transmission system such as this, in order to increase the communication capacity of the entire system, the number of wavelengths can be increased relatively easily. However, if the wavelength band is widened relentlessly, transmission becomes impossible due to limitations of the optical amplification band, the optical fiber transmission band, the optical device band and the like. Therefore, because the wavelength band per se is limited to the most efficient width, the number of wavelengths must be increased by narrowing the distance between wavelengths instead.
In the optical transmitting amplification unit 17 and the optical receiving amplification unit 18, the gain bandwidth of a general multi-wavelength EDFA (Erbium Doped Fiber Amplifier) assigned for each band of L-band, C-band, S-band, and the like is approximately 28 to 32 nm. Therefore, as shown in FIG. 1C, the number of multiplexed wavelengths multiplex varies depending on how many wavelengths are fitted into the range of this gain wavelength band.
At this time, as a factor interfering with the increase in the number of wavelengths, the accuracy of each light sources wavelength becomes an issue. As shown in FIG. 1A and 1B, if optical signals are generated by installing the light source of each wavelength independently into each transmitting unit, an error of Δλ cont occurs in the self-sustaining oscillation accuracy of each wavelength, as shown in FIG. 1D.
In addition, unsurprisingly production tolerance issues occur in the transmission characteristics of the optical device (wavelength filter) implemented as the wavelength multiplexing unit 15 or the wavelength separation unit 16, such as an Arrayed Wavelength Grating (AWG), for example.
For example, the transmission characteristics when WDM light is incident on port P3 of a wavelength filter as in FIG. 1E and optical signals with a wavelength of λ1 and λ2 are respectively output from port P1 and port P2 are as shown in FIG. 1F. In FIG. 1F, the curved line 31 indicates light attenuation from port P3 to port P1, and the curved line 32 indicates light attenuation from port P3 to port P2. In order to separate these optical signals using the wavelength filter, λ1 and λ2 must be Δλ filter apart, after taking production tolerance into consideration.
Furthermore, under the presumption that the optical spectrum widens by Δλ mod due to modulation, the following condition applies to the wavelength separation Δλ of λ1 and λ2.Δλ>Δλcont+Δλfilter+Δλ mod  (1)
In this way, if the factors production tolerance in the wavelength accuracy and wavelength filter of the light source are taken into consideration, it becomes clear that there are limits to the method of narrowing the distance between wavelengths. On the other hand, a method for increasing the number of wavelengths without narrowing the distance between wavelengths by widening the optical amplification band using Raman amplification technology is also being considered.
In addition, if the number of wavelengths increases, it becomes necessary to prepare the same number of laser oscillators which emit light of accurately differing wavelengths as the number of wavelengths with adequate intervals between wavelengths, and the cost for this section will make up the majority of the cost of the entire system.
Therefore, in order to support the significant increase in communication capacity, it is effective to reconsider the construction of the light source and reduce costs. One method for this can be providing multi-wavelength light to a plurality of stations from multi-wavelength light sources.
FIG. 1G is a block diagram of a WDM transmission system using a multi-wavelength light source such as this. The WDM transmission system in FIG. 1G has a construction wherein the transmitting units 11-1 to 11-20 in the construction in FIG. 1A are replaced with transmitting units 42-1 to 42-20, wavelength separator 41-1 to 41-4 are added to station A to station C, an optical coupler 43 is added to station B, and furthermore, a station D is added.
Transmitting units 42-1 to 42-20 have, as shown in FIG. 1H, a construction wherein the light source 21 is eliminated from the construction of FIG. 1B, and optical signals are generated by modulating light input externally with transmission data string. Station D comprises a multi-wavelength light source providing device 44, and multi-wavelength light which is continuous wave (CW) light comprising light of multiple-wavelengths is provided to station A to station C. The optical coupler 43 of station B divides the provided multi-wavelength light into two and outputs the lights to wavelength separators 41-2 and 41-3, respectively.
At station A, the wavelength separator 41-1 separates the provided multi-wavelength light into light of each wavelength, and outputs it to transmitting units 42-1 to 42-5. In the same way, the wavelength separators 41-2 to 41-4 of station B and station C also fulfill the role of separating the multi-wavelength light provided by the multi-wavelength light source providing device 43 in to light of each wavelength.
Multi-wavelength light generated by one multi-wavelength light source can maintain a separation between wavelengths even after passing through wavelength separators 41-1 to 41-4. Therefore, there is no need to take into consideration the oscillation accuracy error of Δλ cont, described above. In addition, because it is not necessary to have a laser oscillator for each transmitting unit, the cost of the light source section of the system as a whole can be reduced.
Furthermore, in recent years, multi-wavelength batch conversion technologies, such as those represented by the commercialization of Photonic Crystal Fiber (PCF) and Periodically Poled Lithium Niobate (PPLN) as multi-wavelength conversion element, are being developed. The usage of these new technologies is undeveloped territory, and future market expansion is expected.
However, in the WDM transmission system utilizing the fore-going multi-wavelength light source, the following issues exist.
In the WDM transmission system utilizing the multi-wavelength light source in FIG. 1G, it is necessary to provide a separate optical fiber for multi-wavelength light provision, in comparison to the conventional WDM transmission system in FIG. 1A.
However, if in fact there is no suitable optical fiber, in order to use the wavelength already used in station A in station B, the only method would be to assign the light source of this wavelength to the transmitting unit of station B and modulate, separately, as with station B in the construction in FIG. 1A.
For example, as shown in FIG. 1I, if four stations, station A to station D, are positioned in a ring configuration, station A can separate and modulate the multi-wavelength light provided from the multi-wavelength light source providing device 51 into light of each wavelength and transmit each wavelength to the adjacent lower station B or station C. However, if the lower station which received the modulated light uses the same wavelength, the light source for this wavelength must be provided within this station.
In addition, as shown in FIG. 1J, if three stations, station A to station C, are positioned in a back to back configuration, station A and station C on both ends can separate and modulate the multi-wavelength light provided from the multi-wavelength light source providing devices 61 and 62 into light of each wavelength and transmit them to the adjacent lower station B. However, if the lower station B which received the modulated light uses the same wavelength, the light source for this wavelength must be provided within this station.
However, if the light source is provided independently of the multi-wavelength light source providing device, it becomes necessary to design taking into consideration the production tolerance of this light source, as stated above, and in comparison to a system such as that in FIG. 1G, measures such as further narrowing the separation between wavelengths become difficult.